1. Field of the Invention
The present invention relates to a film thickness measuring method and a film thickness measuring apparatus using fluorescence which is applicable to measurement of the film thickness of an organic electroluminescence device (hereinafter referred to as an organic EL device).
2. Description of the Related Art
Organic EL devices are rapidly coming into practical use, after an organic light emitting device having characteristics such as high luminance, low voltage drive, small size and high efficiency was published in Applied Physics Letters [51(12), pp. 913 to 915 (1987)] in 1987 by C. W. Tang and S. A. VanSlyke. Tang et al. formed an ultra-thin film of an organic dye, thereby increasing the efficiency of the conventional organic light-emitting device by a factor of 10. Note that the organic dye they used can easily be processed into an amorphous film. According to this report, an external quantum efficiency of 1%, a visual luminous efficiency of 1.5 lm/W and a luminance of 1000 cd/m2 are achieved with a driving voltage of 10 V or less. Furthermore, the reduction of voltage is achieved by using a magnesium silver alloy having a relatively small work function as a cathode. Ten years have passed since the above report, and present organic EL deices have a higher efficiency and longer life, and matrix panels are also commercially available.
The organic EL device has such characteristics as spontaneous luminance, thinness, low weight, high luminance and high efficiency, and is expected as a next-generation flat-panel display. The basic configuration of an organic EL device is shown in FIG. 1. In the organic EL device of FIG. 1, a transparent electrode 2 made of an electrode material transparent in a visible region such as ITO (Indium Tin Oxide) is formed on a transparent substrate 1 such as glass, quartz or a plastic film. This transparent electrode 2 has a thickness of about 100 nm, for example. On the surface of the transparent electrode 2, there are deposited a hole transport thin film 3 of about 50 nm made of a hole transport thin film material, and an electron transport light emitting thin film 4 of about 50 nm made of an electron transport light emitting thin film material. In addition, an upper electrode 5 having a thickness of about 50 nm to 200 nm is formed on the surface of the electron transport light emitting thin film 4. The organic EL device of FIG. 1 has a double layer structure, but recently, devices having multi layer structures, such as a three or four layer structure have also been developed.
As shown in FIG. 1, the organic EL device has permeability in at least one of the electrodes in the visible region, and has a configuration having an ultra thin organic film of about 100 nm (e.g., 3 and 4 of FIG. 1) between the electrodes 2 and 5 of the device. Its driving voltage characteristics largely depend upon the thickness of the organic thin film, meaning that obtaining uniformity in thickness of the organic thin film within the surface has been a significant challenge. This is because nonuniformity of the film thickness caused during formation of the organic thin film could lead to a reduced yield. If a film thickness distribution can be measured, especially during film formation, and is fed back to a film forming apparatus, improvements in the uniformity of the film within the surface, will result, and the yield will be raised, reducing costs.
To date various methods and apparatuses have been used to measure the thickness during formation or after formation of an organic thin film in an organic EL device. For example, in the organic EL device of FIG. 1, the thickness may need to be measured during formation or after formation of the hole transport thin film 3, and during formation or after formation of the electron transport light emitting thin film 4 thereon. Regarding the principal methods of measuring the film thickness, a quartz oscillator method is widely used during film formation in vacuum, and a film thickness measuring method using light interference in the film after formed, as they utilize nondestructive and non-contact methods. Further, when a film is formed by screen printing, spraying or application, the film thickness is normally measured by use of, for example, light interference after film formation.
To put it simply, the principle of measurement in accordance with the quartz oscillator method is that the natural oscillation of the quartz oscillator changes along with its mass change. In other words, it utilizes the fact that when a thin film is deposited on the quartz oscillator, the same effects are simply produced as those of increased mass or thickness of the quartz oscillator if the mass of the thin film is sufficiently lower than the mass of the quartz oscillator, thus producing a natural oscillation frequency change proportionate to the mass change.
In the film thickness measuring method using light interference, when the film is irradiated with a constant wavelength light, luminous flux which has permeated through the film, has been reflected on the bottom surface and returned up to the surface and luminous flux reflected on the film surface cause interference. By measuring prisms that have caused the interference and analyzing upper and lower peak positions of the interference, the film thickness can be obtained.
However, the above quartz oscillator method is a method in which the thickness of the organic thin film on the transparent substrate is not directly observed, and the film thickness is estimated from deposits accumulated on the quartz oscillator set in the same vacuum device. It is thus essentially impossible to directly measure the thickness of the organic thin film formed on the transparent substrate. Therefore, it is impossible to measure the film thickness distribution of the organic thin film formed for use in, for example, flat panel displays, in a nondestructive manner during a production process. Further, such a disadvantage is caused that, when used simply to monitor the film thickness during a production process, the quartz oscillator must be replaced, if the natural oscillation frequency of the quartz oscillator is significantly decreased due to accumulated deposits.
On the other hand, when the film thickness measuring method using light interference is used, the film thickness distribution of the organic thin film on the substrate can be nondestructively measured, but the problem is that this is extremely complicated.